The above list includes all the permissible explosives that have passed the required tests prior to January 1, 1912. The announcement of the passing of like tests by other explosives will be made public after the completion of the tests. RESULTS OF RECENT TESTS. During the past year 25 permissible explosives have been withdrawn by the manufacturers of explosives, and 31 new explosives have been added to the permissible list. All explosives that are no longer being manufactured, as well as those that have shown unfavorable results in field tests, have been eliminated from the list. In the ballistic pendulum test for determining the power of an explosive, the standard deflection of the pendulum is that produced by one-half pound (227 grams) of 40 per cent nitroglycerin dynamite. Some explosives do not produce an equal deflection unless they are used in charges of 1 pound (454 grams) or more. It has been found that such explosives must be used in charges of more than 1 pounds in order to satisfactorily break down coal. It is believed that in gaseous or dusty coal mines no explosive can be used safely in quantities greater than 11⁄2 pounds per shot. Consequently, all explosives that, under normal conditions of mining, can not be satisfactorily used in charges of 14 pounds or less have been eliminated from the permissible list. It has been found further that all explosives that on detonation evolve 5 cubic feet (158 liters) or more of permanent poisonous gases from a 14-pound charge as determined by gage tests, when used in 14-pound quantities in narrow entries in coal mines, ahead of the air, may for several minutes after a shot so vitiate the air near a face as to be harmful to the health of those directly connected with the work. It is true that the conditions mentioned above are unfavorable to any explosive under test and are abnormal for the reason that there is no circulation of air near the face. However, as such conditions actually exist in some coal mines, all explosives that produce 5 cubic feet (158 liters) of poisonous gases from a 14-pound charge have been eliminated from the permissible list. A new test of explosives for determining their sensitiveness to frictional impact (a glancing blow) has been adopted by the Bureau of Mines. The apparatus used comprises a steel anvil and a swinging steel shoe. The anvil has a smooth face, 34 inches wide by 12 inches long, in the middle of which are three grooves for holding the charge of explosive. The shoe has a radius of swing of 6 feet 6 inches and the face of the shoe has a radius of curvature of 10 inches. The shoe can be faced with hardwood fiber or other material. The shoe may be dropped from heights of 19.7 to 78.7 inches (one-half to 2 30530°-12-2 meters) and weights of 2.2 to 44 pounds (1 to 20 kilos) are used in the tests. The test was devised to represent what may happen in mining when a cartridge of explosive becomes lodged in a drill hole and must be forced home by ramming. Some explosives have been found to be extremely sensitive to the frictional impact test even when the shoe is faced with wood fiber. Consequently great care should be exercised with all explosives in charging drill holes. Results of tests made in the gallery indicate that no explosive is safe for "adobe shots" a in breaking up large pieces of rock or coal or in breaking down stoppings in coal mines where either gas or dry inflammable dust is present in quantities or under conditions that indicate danger. Black blasting powder, even when covered with large quantities of clay, can not be safely or effectively used for this work. Ordinary dynamites are equally unsafe, even when put in a bucket of water placed on the material to be broken; they give sufficient flame to cause the explosion of an inflammable mixture of gas and air. Permissible explosives have been found to be safer than black blasting powder or ordinary dynamite for use in "adobe shots," but the charges used must be small and must be well covered with moist clay. However, all permissible explosives when fired under these conditions in the presence of a mixture of gas and air containing 8 per cent of gas (methane) are likely to cause ignition and consequently "adobe shots" should never be fired in the presence of fire damp. In emergencies, such as in fighting mine fires or in recovery work when "adobe shots" are absolutely necessary, the working place should be carefully tested for gas before firing. The charge used should not exceed 14 pounds and it should be well covered with moist clay. The underlying reasons why one explosive passes and another fails when tested in the presence of inflammable gas and dust have been investigated at the Pittsburgh testing station. The results of such researches have been reported to the manufacturers, and after their explosives have failed to pass tests they have frequently so changed and perfected their explosives that later when new explosives were submitted all the requirements of the bureau have been satisfactorily met. The results of tests indicate that any explosive fired in very large quantities will cause ignition of inflammable gas and coal dust mixtures. An arbitrary charge, namely, 14 pounds, has been established as the maximum amount of explosive to be used in making tests, and all explosives in order to be placed on the permissible list must pass a For an "adobe shot" the explosive is placed on the thing to be broken and is covered with clay. This method of using an explosive is also called "bulldozing." the gas and dust test with this charge. A charge of 14 pounds of explosive per drill hole should never be exceeded in practice; in good mining practice the charge need not exceed 1 pound, an amount which gives a greater factor of safety. Explosives of many different compositions are now on the permissible list, but every one is designed to give on detonation a relatively low flame temperature and a flame of short duration. To ignite inflammable gas and coal-dust mixtures a certain temperature, acting through a certain length of time, is required. The flame temperature of all explosives exceeds the ignition temperature of inflammable gas and dust mixtures, but fortunately the flame of a permissible explosive properly detonated is of such short duration that it does not ignite the inflammable mixtures. Manifestly, any factor that increases the duration of the flame temperature of a permissible explosive increases the danger in the use of that explosive. Such danger is present when an improper detonator is used or when any explosive is not used in accordance with the provisions prescribed by the Bureau of Mines. CLASSES OF PERMISSIBLE EXPLOSIVES. In order that the users of explosives may know the nature and characteristic component of each of the permissible explosives, and as an aid in the selection of an explosive to meet a specific requirement, Table No. 2 has been prepared. In this table the permissible explosives are arranged in four classes, and the rate of detonation of each explosive is given. |